Zachary at RPG Blog II is creating a series of posts on Rolemaster for 2010. Rolemaster, specifically Arms Law, was first developed as an extension of D&D. His latest post got me thinking on how to apply some of that to Swords & Wizardry and other retro-clones.
The first thing, other than copyright issues, is that as fun the Rolemaster tables are using them or something similar probably not appealing to the majority of old school fans. But following simple rule can capture much of the deadliness that the open ended critical roll gives.
When a natural 20 is rolled the attacker add maximum damage to his normal damage result. If he rolls another 20 he adds double his maximum damage, another 20 triple and so on until he fails to roll a 20.
For example Stump the Dwarf see an elder demon, fearing for his life he throws his battle axe. Rolling to hit he gets a natural 20, he roll again getting another 20, then yet another 20, finally is last roll is a 11. Doing 1d8+1 with his battle axe he does 8+16+24+1d8+1 damage or 45+1d8. Taking a fearsome blow the demon flees.
Like my ritual rule this changes the tone of a D&D game so it may not be for all campaigns. It give a more gritty and deadlier feel to combat. It is considerably less complicated than the rolemaster charts while giving some of the same deadliness. I think it will work better for Swords & Wizardry, both the White Box and the Core edition, with it's lower hit points totals.